Familial aggregation may be due to genetic or shared environmental factors. Twin studies can be used to estimate the contribution of inheritance to the familial aggregation of a trait or disease by comparing the similarities (concordance rate) of a trait or disease in monozygotic (MZ, genetically identical) and dizygotic (DZ, sharing on average 50% of genetic material) twins. In a registry of 4840 male twin pairs in Sweden, 458 cases of prostate cancer were identified. There were 16 concordant pairs among 1649 MZ twin pairs (1%) but only six concordant pairs among 2983 DZ twin pairs (0.2%). The greater concordance rate seen in MZ twins compared to DZ twins is attributed to a greater degree of shared genes.2 A second twin study, conducted in the United States, showed similar results.4 Among 1009 twin pairs identified from a national twin registry, a significantly higher concordance rate was observed among MZ twins (27.1%) compared to DZ twins (7.1%). It was estimated that genetic influences account for approximately 57% and environmental influences for 43% of the variability in twin liability for prostate cancer. In the twin study conducted by Lichtenstein et al.,6 a 21% concordance rate was found in MZ twins but only a 6% concordance rate in DZ twins. It was estimated that 42% [95% confidence interval (CI) = 29%-50%) of prostate cancer risk was due to heritable factors, and this genetic contribution was the highest among all neoplasms studied.
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